The present invention relates to a method for growing nitride semiconductor crystals, a nitride semiconductor device and a method for fabricating the same.
Nitride semiconductors such as GaN, InN and AlN are materials suitably used for blue-light-emitting semiconductor laser devices and numerous types of semiconductor devices, e.g., transistors operating at a high speed at an elevated temperature.
Various methods have been suggested to form a single crystal layer of a nitride semiconductor suitable for these semiconductor devices.
For example, according to a conventional technique, a nitride semiconductor layer (e.g., an AlN layer) is directly deposited on a single crystal substrate of sapphire (Al2O3) or Si by a metalorganic vapor phase epitaxy (abbreviated to xe2x80x9cMOVPExe2x80x9d and also called a xe2x80x9cmetalorganic chemical vapor deposition (MOCVD)xe2x80x9d) process. The nitride semiconductor layer formed by this method, however, has poor surface morphology and is likely to crack, resulting in a lower yield. Thus, this method has not been put into practice. Cracking is probably caused due to a thermal stress resulting from a difference in thermal expansion coefficient between a single crystal substrate and a nitride semiconductor layer during the process of lowering the deposition temperature of the nitride semiconductor layer (about 1000xc2x0 C. for AlN) to room temperature.
Another technique of forming a single crystalline nitride semiconductor layer was developed later as disclosed in Japanese Laid-Open Publications Nos. 4-297023 and 7-312350. According to this technique, an amorphous or polycrystalline nitride semiconductor layer (i.e., a GaN or Ga1xe2x88x92aAlaN (where 0 less than axe2x89xa61) layer) is once formed on a single crystal substrate of sapphire or silicon at a relatively low temperature by an MOVPE process. Thereafter, the nitride semiconductor layer is heated to form a partially single crystalline buffer layer and then nitride semiconductor layers for a semiconductor device are epitaxially grown on the buffer layer.
A light-emitting device disclosed in Japanese Laid-Open Publication No. 6-177423 is known as an exemplary semiconductor device using a nitride semiconductor layer formed on a buffer layer. As shown in FIG. 14, this light-emitting device 900 includes: a buffer layer 95 of polycrystalline or amorphous GaN or Ga1xe2x88x92aAlaN (where 0 less than axe2x89xa61); an n-type Ga1xe2x88x92bAlbN (where 0xe2x89xa6b less than 1) cladding layer 96; an n-type InxGa1xe2x88x92xN (where 0 less than x less than 0.5) active layer 97; and a p-type Ga1xe2x88x92cAlcN (where 0xe2x89xa6c less than 1) cladding layer 98, which are stacked in this order on a sapphire substrate 92.
The crystal growing technique for the buffer layer 95 is also disclosed in Japanese Laid-Open Publications Nos. 4-297023 and 7-312350 identified above. Specifically, according to the method disclosed in these references, GaN or Ga1xe2x88x92aAlaN (where 0 less than axe2x89xa61) crystals are grown at a temperature ranging from 200xc2x0 C. to 900xc2x0 C., both inclusive, by an MOVPE process to form the buffer layer 95. In accordance with this method, part of the buffer layer 95 is turned into single crystals during a process of raising the temperature after the buffer layer 95 of polycrystalline Ga1xe2x88x92aAlaN (where 0 less than axe2x89xa61) has been deposited on the sapphire substrate 92 at a low temperature and before a nitride semiconductor crystal layer, e.g., the n-type Ga1xe2x88x92bAlbN (where 0xe2x89xa6b less than 1) cladding layer 96, is deposited at a temperature of about 1000xc2x0 C.
The present inventors minutely analyzed the cross-section of nitride semiconductor crystals, which had been grown on a sapphire substrate at a low temperature by the conventional technique, using a transmission electron microscope. As a result, we found that the nitride semiconductor crystal layer, which had been formed by the prior art crystal growing technique, had a lot of dislocations and that the lifetime of a semiconductor device including such a nitride semiconductor layer was short.
In the conventional method for fabricating a semiconductor device, it seems to be only a small region of the buffer layer 95 within a plane of the sapphire substrate 92 that is turned into single crystals during the temperature raising process before the nitride semiconductor crystal layers are grown. Thus, it is considered that, in the remaining region of the buffer layer 95 that is not turned into single crystals, the polycrystals have poorly aligned orientations to generate a large number of dislocations (or other defects) in the interface between the sapphire substrate 92 and the buffer layer 95. And such dislocations would grow to reach the nitride semiconductor crystal layers (i.e., the cladding layer 96, active layer 97 and cladding layer 98 in this case). We found that the density of dislocations in the nitride semiconductor crystal layers was as high as 109 cmxe2x88x922, thus adversely shortening the life of the semiconductor device.
Still another technique of forming an AlN buffer layer by nitrifying (in this specification, to xe2x80x9cnitrifyxe2x80x9d means xe2x80x9cto combine with nitrogen or its compoundsxe2x80x9d) the surface of a sapphire single crystal substrate was suggested in Japanese Laid-Open Publication No. 63-178516, for example. In accordance with this technique, however, the buffer layer is also likely to crack or a lot of dislocations are also created in the buffer layer as in the prior art method just described. Thus, this technique has not been put into practice, either.
An object of the present invention is providing a method for growing nitride semiconductor crystals with the number of dislocations created in a nitride semiconductor crystal layer reduced, a highly reliable semiconductor device with a longer lifetime, and a method for fabricating the same.
A method for growing nitride semiconductor crystals according to the present invention includes the steps of: a) forming a first metal single crystal layer on a substrate; b) forming a metal nitride single crystal layer by nitrifying the first metal single crystal layer; and c) epitaxially growing a first nitride semiconductor layer on the metal nitride single crystal layer.
The present invention also provides a method for fabricating a nitride semiconductor device including a semiconductor multilayer structure and a pair of electrodes for applying a voltage to the semiconductor multilayer structure. In this method, the step of forming the semiconductor multilayer structure includes the step of epitaxially growing the first nitride semiconductor layer by the method of the present invention for growing nitride semiconductor crystals.
A nitride semiconductor device according to the present invention includes: a single crystal substrate; a metal nitride single crystal layer formed by nitrifying a metal single crystal layer on the single crystal substrate; a semiconductor multilayer structure including a first nitride semiconductor layer epitaxially grown on the metal nitride single crystal layer; and a pair of electrodes for applying a voltage to the semiconductor multilayer structure.
Another nitride semiconductor device according to the present invention includes: a single crystal substrate with conductivity; a metal nitride single crystal layer formed by nitrifying a metal single crystal layer on the single crystal substrate; a semiconductor multilayer structure including a first nitride semiconductor layer epitaxially grown on the metal nitride single crystal layer; and a pair of electrodes formed to face each other on respective surfaces of the single crystal substrate and the semiconductor multilayer structure, which are interposed between the surfaces.